In the industry, there is frequent demand for producing a mixture of powder or granular state material and liquid. The mixing process must ensure hydration on the entire surface of large specific surface grains. In these applications, mixing with a liquid frequently causes the transformation of powder or granular state material as well, i.e. its chemical and physical characteristics are modified. The extent of mixing and thus that of the transformation are determined by the quantity of water added and by the technology applied for mixing.
At the mixing in these procedures, from the aspects of carrying the mixture through a pipeline, the specific water consumption and the transformation of powder or granular state material, it is usually advantageous if the powder or granular state material is mixed with a liquid of nearly identical amount as the weight of the material, thereby producing a mixture of high density, approx. 1/2-2/1 solid/liquid weight ratio.
Such an application is for example the producing of so-called dense slurry (also called as thick slurry) consisting of a solid phase combustion residue from a coal or petcoke fired power station and water at solid/liquid weight ratio of approx. 1/2-2/1, typically at 1.5-1 solid/liquid ratio. When producing dense slurry, it is important to ensure the mixing as good as possible, because it is desirable that particles of free calcium-oxide (CaO) content adhered to the fly ash grains, settled in the porous parts of the fly ash grains are fully dissolved. Through its so-called puzzolanic activity, the combustion residue of high specific surface would not or only slightly exhibit a cementing characteristic on its own, but in the presence of moisture and at a normal temperature, the materials of combustion residue enter into a chemical reaction with calcium-hydroxide (Ca(OH)2) through mixing and due to this chemical reaction, they are converted to a solid structure material. Intensive mixing ensures a rapid reaction rate. In this way, the dense slurry becomes solid after being delivered along in a pipeline and after being disposed. As a result of appropriate mixing, after solidification, a high volumetric density, high compression strength and low water permeability deposit is created.
In case of dense slurry, the amount of water added to the solid material is significantly less than in the case of the lean slurry (also called as thin slurry). In this context, the chemical reactions of the solid material and the water play an essential role in case of dense slurry as compared to lean slurry.
If, by a method, dense slurry is produced as a material mixture, then various materials produced during the manufacturing processes can be distinguished, such as                Product phase I—the material directly after mixing and delivery (transport), the dense slurry itself;        Product phase II—the deposited material after a few days of storage, after surface water has leaked/evaporated and cementation process has started;        Product phase III—the deposited material after longer storage (typically few months), after the process of cementation has practically been completed: deposit. (In the material certain additional mineral transformations may take longer).        
The binding characteristics of the material basically depend on the mineral composition of the raw material used in the method as a base material, where relevant factors are the reactive (free) CaO/MgO content, and their proportion to the silicate/aluminate components. The binding characteristics furthermore depend on the mixing process itself, where the mixing ratio by weight (solid/water) and the intensity of mixing are also significant factors. The mixing water quality has also some relevance.
WO 98/08596 A1 describes an apparatus adapted for producing a mixture, e.g. dense slurry. The known apparatus comprises a mixing space, a hopper for introducing the powder or granular state material into the mixing space, an inlet pipe for introducing the liquid into the mixing space, a pipe for re-circulating a part of the mixture into the mixing space, and an outlet pipe for delivering the mixture.
In the known apparatus, an upper mixing space and a lower mixing space are arranged one below the other, which are connected to each other with a conducting pipe having a cross section narrower than that of the upper mixing space. The apparatus has a first circulating circuit for delivering one part of the mixture accumulated in the bottom part of the lower mixing space into the upper mixing space and a second circulating circuit for delivering another portion of the mixture accumulated in the bottom part of the lower mixing space into the upper part of the lower mixing space.
In the light of the known solutions, especially the apparatus described in WO 98/08596 A1 there is a need for a method for producing a mixture, especially a dense slurry, consisting of powder or granular state material and liquid, which production method with the help of the apparatus disclosed in WO 98/08596 A1 could produce a mixture of appropriate quality, free of excessive air content, and having a uniform quality.